This invention relates to nonlinear optical crystals, and more particularly to the conversion of optical radiation having a first frequency to optical radiation having a second frequency using nonlinear optical crystals.
There are many known sources of optical radiation, which can be characterized by a frequency, or frequency spectrum. A method of generating optical radiation of a desired frequency is to generate optical radiation of a first frequency, different from the desired frequency, and then to convert this to optical radiation having the desired frequency. For example, a pump laser can generate optical radiation having a frequency xcfx89 (i.e., fundamental frequency). This optical radiation can then be converted to optical radiation having a frequency 2xcfx89 (i.e., harmonic frequency) by appropriate illumination of a nonlinear frequency doubling crystal with the optical radiation having frequency xcfx89.
The conversion efficiency of a pump laser beam into its harmonics is generally low. The power of a harmonic beam is related to the power of the fundamental pump beam in a nonlinear way. Hence it is not uncommon for high power pump lasers to be tightly focused onto a nonlinear crystal in order to generate sufficient power in the harmonic. For example, a 25 watt (W) pump laser may be focused onto a nonlinear crystal to power densities of about 250,000 W/cm2, generating about 10 to 20 milliwatts of power in the frequency doubled output beam.
The high power densities of pump laser beams in these systems can locally damage the nonlinear crystal. This, in turn, can lead to degradation of the power levels of the output beam. For example, in some cases the damage to the nonlinear crystal results in increased absorption of the pump beam by the nonlinear crystal.
A technique commonly used to overcome undesirable degradation of the power levels of the output beam is to vary the area of the nonlinear crystal on which the pump beam is focused. This can be achieved, for example, by translating the nonlinear crystal. In addition, increasing the power of the fundamental pump wavelength can compensate for absorption losses. However, the amount of additional power available may be limited and will depend on the laser source.
In applications where high pump beam power is required to maintain sufficient harmonic output power, the pump lasers used are typically large, complex, expensive systems, demanding expensive utilities (e.g., 3-phase power, flowing cooling water and high purity nitrogen). Such pump lasers are limiting in applications having space, utility, and/or budget constraints.
The invention features a nonlinear optical crystal assembly and a gas mixture that surrounds the nonlinear crystal. The gas mixture reduces photochemical degradation of the nonlinear crystal caused by exposure of the nonlinear crystal to a high power light source. The assembly may be incorporated into a light source, and applications requiring a light source, such as, e.g., applications requiring ultraviolet light. In some embodiments, the nonlinear crystal assembly may be placed inside an optical cavity. Generally, the nonlinear crystal converts optical radiation from a pump source having a first frequency, to optical radiation having a second frequency, different from the first frequency.
In general, in one aspect, the invention features an optical system including: a light source providing a pump beam having a first frequency; a nonlinear optical crystal positioned to transform at least a portion of the pump beam into an output beam having a second frequency different from the first frequency; and an enclosure filled with gas and surrounding the nonlinear optical crystal, the gas including hydrogen and oxygen in amounts sufficient to reduce photochemical degradation of the nonlinear optical crystal caused by the pump beam. In some embodiments, the gas is sealed within the enclosure. In other embodiments, for example, the system further includes a gas source coupled to the enclosure for flowing the gas into the enclosure.
In general, in another aspect, the invention an optical system including: a light source providing a pump beam having a first frequency; a nonlinear optical crystal positioned to transform at least a portion of the pump beam into an output beam having a second frequency different from the first frequency; an enclosure surrounding the nonlinear optical crystal; and a gas source of hydrogen and oxygen coupled to the enclosure, wherein during operation the gas source provides the enclosure with amounts of hydrogen and oxygen sufficient to reduce photochemical degradation of the nonlinear optical crystal caused by the pump beam.
Embodiments of either optical system may include any of the following features.
The enclosure may surround the nonlinear optical crystal and the light source.
The system may further include a plurality of mirrors defining an optical cavity surrounding the nonlinear optical crystal. For example, the optical cavity may be resonant at the first frequency. Furthermore, the light source may located within the optical cavity. Moreover, the light source may include a gain medium and the optical cavity may resonantly enhance emission from the gain medium to generate the pump beam. For example, the light source may include a gas tube (e.g., an Argon ion gas tube) and electrical source coupled to the gas tube, and wherein during operation the electrical source produces an ion discharge in the gas tube. The gas tube may be air-cooled. Alternatively, the light source (e.g., a single frequency laser) may be located outside of the optical cavity, and wherein during operation the light source couples the pump beam at the first frequency into the optical cavity. In either case, the enclosure may also surround the optical cavity.
The nonlinear optical crystal may include Boron and Oxygen, for example, it may be one of Barium Beta Borate, Lithium Triborate, and Cesium Lithium Triborate.
The second frequency may be a harmonic of the first frequency. For example, the second frequency may be in the UV portion of the electromagnetic spectrum.
The gas including hydrogen and oxygen may further include a buffer gas, such as, for example, Argon or Nitrogen. The ratio of hydrogen to oxygen in the enclosure gas may about one to one. Furthermore, the gas including hydrogen and oxygen may have a hydrogen concentration of less than or equal to about 10%. Also, the gas including hydrogen and oxygen may have an oxygen concentration of less than or equal to about 10%. Furthermore, both the hydrogen and oxygen may have a concentration of less than or equal to about 10%. Similarly, the respective concentrations may be less than or equal to about 3%, and may be as low as about 0.1%. The hydrogen may include, e.g., hydrogen molecules or hydrogen ions. The oxygen may include, e.g., oxygen molecules, oxygen ions, or ozone. Furthermore, for example, the gas may include about 95% Argon, about 2.5% oxygen, and about 2.5% hydrogen. The concentration refers to the partial pressure concentration of the respective gases.
Furthermore, the gas in the enclosure may have a pressure greater than ambient pressure (i.e., greater than about 1 atmosphere), for example, the gas pressure may be greater than the ambient pressure by an amount up to 10 Psi.
The optical system may further include a heating element thermally contacted to the nonlinear optical crystal and a temperature controller coupled to the heating element.
For example, during operation the temperature controller may cause the temperature of the nonlinear optical crystal to be at least 50xc2x0 C., or to be at least 70xc2x0 C.
The light source may be an Argon ion laser, a Krypton ion laser, a YAG laser, or an Alexandrite laser, or it may include the corresponding gain medium when the system includes an optical cavity and the light source is positioned within the cavity. The light source may be a continuous wave laser. The light source may be an air-cooled laser.
In another aspect, the invention features an optical microscopy system including: either of the optical systems described above; and a microscope positioned to receive the output beam from the optical source.
Furthermore, in general, in another aspect, the invention features an nonlinear optical crystal assembly including: a nonlinear optical crystal positioned to transform at least a portion of a pump beam having a first frequency into an output beam having a second frequency different from the first frequency; and an enclosure filled with gas and surrounding the nonlinear optical crystal, the gas including hydrogen and oxygen in amounts sufficient to reduce photochemical degradation of the nonlinear optical crystal caused by the pump beam.
Furthermore, in general, in another aspect, the invention features a nonlinear optical crystal assembly including: a nonlinear optical crystal positioned to transform at least a portion of a pump beam having a first frequency into an output beam having a second frequency different from the first frequency; an enclosure surrounding the nonlinear optical crystal; and a gas source of hydrogen and oxygen coupled to the enclosure, wherein during operation the gas source provides the enclosure with amounts of hydrogen and oxygen sufficient to reduce photochemical degradation of the nonlinear optical crystal caused by the pump beam.
Either of the nonlinear optical crystal assemblies may include any of the corresponding features described above for the optical systems.
In general, in another aspect, the invention features an optical method including: directing a pump beam having a first frequency to a nonlinear optical crystal positioned to transform at least a portion of a pump beam into an output beam having a second frequency different from the first frequency; and surrounding the nonlinear optical crystal with a gas including hydrogen and oxygen in amounts sufficient to reduce photochemical degradation of the nonlinear optical crystal caused by the pump beam. Embodiments of the method may include any of the corresponding features described above for the optical systems.
Embodiments of the invention may include any of the following advantages:
Photodegradation of the nonlinear crystal can be reduced. As a result, the crystal may be used to efficiently produce a stable output beam from a nonlinear interaction in which one or more beams are directed to the crystal. For example, the power of a harmonic output beam generated by the interaction of a pump beam and the nonlinear crystal may remain substantially constant with the pump beam focused continuously on the same area of the nonlinear crystal and maintaining the pump beam at a substantially constant power. Moreover, reducing the photodegradation may reduce amplitude noise fluctuations in the output radiation.
Furthermore, a laser of reduced power and complexity may be used as a pump beam source in applications requiring substantial power at harmonic frequencies. For example, a system used to generate ultraviolet light may use an air-cooled pump laser to pump a nonlinear crystal. The output power of such a source may be in the range of at least milliwatts to tens of milliwatts.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.